I don't know if this will be of help to others but its worth putting here in case others experience similar.

I've had issues with my beer slowly souring in the keg, my dispensing set up from a sankey keg was a Pegas Ecojet counter pressure bottle filler which has a connection for a Perlick type tap, which I bought a chinese copy from Ali Express.

The whole set up was flushed with line cleaner regularly and stripping down the tap and cleaning it didn't rectify matters. So I've swapped those out in favour of a Dalex tap and I'm experiencing no issues now; it would seem that somewhere within that setup infection was able to pass back through the pipes into the keg.

I suspect strongly that its the tap that was the issue rather the Pegas filler as adding a cap to the end of the tap slowed the souring. I shall use the Pegas as and when I want to bottle some, but am pleased that swapping out the tap has worked.

we're pretty lazy on cleaning lines n such, so using the same line and tap after a wild yeast beer was interesting putting a pale back on: you'd pour a glass each and get almost totally different tasting beers. the scent even was an immediate giveaway to which was poured first - real heavy brett flavours. interestingly that second glass and the rest of the session was clean english ale so the contamination seemed limited to the beer in the line. how quickly did you find the keg went bad?

By "soured" did you mean turned to vinegar? Which should be obvious because the acetic smell/taste is difficult to confuse with anything else.

I don't think this would fit your description as turning to vinegar needs another component to make it happen - oxygen. So it shouldn't creep the length of the pipe and cause the keg to spoil. I had a problem with beer turning to vinegar in the pipes and dug up lots of useful facts in diagnosing it. It needs oxygen as mentioned (acetic acid is formed by oxidation of alcohol). The oxygen will stream in if you're daft enough, like me, to use silicone tubing as beer-line (silicone tubing is permeable to gases, especially oxygen). And you don't need a bacterial infection by acetobacter or the like; there is something far more insidious and it's infecting everyone's beer - it's called "yeast". Yeast is "diauxic", i.e. will use another source of carbon for aerobic respiration when sugar runs low and yeast's case that other source of carbon is ethanol, which should be plentiful in beer 'cos we like a drop of alcohol.